Supercritical fluids may be used to process a wide variety of materials. Examples of supercritical fluids applications include extractions in supercritical carbon dioxide, the growth of quartz crystals in supercritical water, and the synthesis of a variety of nitrides in supercritical ammonia.
Processes that employ supercritical fluids are commonly performed at high pressure and high temperature within a pressure vessel. Most conventional pressure vessels not only provide a source of mechanical support for the pressure applied to reactant materials and supercritical fluids, but also serve as a container for the supercritical fluid and material being processed. The processing limitations for such pressure vessels are typically limited to a maximum temperature in the range between about 400° C. and 600° C. and a maximum pressure in the range between about 100 megapascals (also referred as “MPa”) and 500 MPa. Conventional pressure vessels, or autoclaves, are commonly equipped with a pressure release mechanism, such as a pressure relief valve or a rupture disk, that automatically releases or vents pressure from inside the pressure vessel if the pressure exceeds a predetermined value. Such pressure release mechanisms increase the safety margin for operation of pressure vessels at high pressures and high temperatures.
Processing material with supercritical fluids often requires a container or capsule that is substantially both chemically inert and impermeable to the solvent and any gases that might be generated by the process. The capsule should also be substantially impermeable to any gases or materials on the outside of the capsule. These capsules are commonly made in the form of cylinders, possessing a wall and two ends disposed opposite each other along the axis of the cylinder. In one approach, the material to be processed, along with a solvent (liquid) that forms a supercritical fluid at elevated temperatures, is introduced into a capsule at low temperature. After the capsule has been sealed and returned to near room temperature, the capsule will possess an elevated internal pressure as dictated by the vapor pressure and temperature of the solvent (liquid) within the capsule. In the case of ammonia at room temperature, the pressure within the capsule is approximately 150 pounds per square inch. This internal pressure can cause deformation, strain, cracks, leaks, and failure of the capsule, particularly for capsules larger than several inches in dimension, and/or when the capsule is fabricated from a soft metal such as silver or gold. Reinforcing members for one or more outer surfaces of the capsule may be provided in order to increase its ability to safely handle moderate internal pressures.
Capsules for use with supercritical fluids in high pressure apparatus are often hermetically sealed, by welding or the like. Consequently, it may not be a possible to incorporate a pressure release value or rupture disk into the capsule or high pressure apparatus, potentially raising questions about safety.
Therefore, there is a need for improved techniques for processing materials in a high pressure apparatus are highly desirable. The present invention fulfills this need, among others.